1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator (TCXO) which is incorporated in devices such as portable telephones, and more particularly to a temperature-compensated crystal oscillator which reduces fluctuation in the oscillated frequency which is caused by an electromagnetic field produced at an antenna.
2. Description of the Related Art
Temperature-compensated crystal oscillators, in which variation in oscillation frequency that arises from the frequency-temperature characteristic of the quartz-crystal unit is compensated, find particularly wide use in devices such as portable telephones used in a mobile environment. In such devices, temperature-compensated crystal oscillators are used as, for example, the reference frequency sources of PLL (Phase Locked Loop) circuits which output the communication frequency signal. In such applications, the occurrence of phase error in the communication frequency or carrier frequency between base station and portable telephone causes breakdowns of communication, and the frequency stability of a temperature-compensated crystal oscillator is therefore crucial.
Referring to FIG. 1, in which is shown an example of the configuration of a temperature-compensated crystal oscillator of the prior art. This temperature-compensated crystal oscillator includes quartz-crystal unit 1 and IC (integrated circuit) 2, crystal unit 1 and IC 2 being accommodated within a container, as will be explained hereinbelow.
Crystal unit 1 uses, for example, an AT-cut quartz-crystal blank. Crystal unit 1 which employs an AT-cut crystal blank has a frequency-temperature characteristic which can be represented by a cubic function having an inflection point in the vicinity of normal temperature, as shown in FIG. 2.
IC 2, on the other hand, is a device in which are integrated voltage-controlled crystal oscillator 3, temperature compensation circuit 4, and AFC (automatic frequency control) input circuit 5 that receives as input an AFC voltage. Voltage-controlled crystal oscillator 3 includes crystal unit 1, oscillation circuit 7 which uses crystal unit 1, and voltage-variable capacitance element 6 which is inserted in an oscillation closed-loop. Crystal unit 1 is provided outside IC 2, i.e., independent of IC 2. Oscillation circuit 7 is provided with an oscillation capacitor (not shown in the figure) which forms a resonance circuit (i.e., the oscillation closed loop) together with crystal unit 1, and an oscillation amplifier (not shown in the figure) which feeds back and amplifies the resonant frequency component of the resonance circuit. As will be explained hereinbelow, a control voltage is applied to the cathode of voltage-variable capacitance element 6.
Temperature compensation circuit 4 is provided with temperature sensor 8 which detects the ambient temperature and outputs the temperature information as an electrical signal, and voltage generation circuit 9 which generates a temperature-compensation voltage based on the signal from temperature sensor 8. The temperature-compensation voltage is a voltage which varies as a cubic function with respect to the ambient temperature. The temperature-compensation voltage is applied to the cathode of voltage-variable capacitance element 6 by way of high-frequency blocking resistor 15. As a result, the equivalent serial capacitance of the oscillation closed-loop varies as seen from crystal unit 1 in accordance with the temperature-compensation voltage, the oscillation frequency of crystal oscillator 3 varies, the temperature dependence of the oscillation frequency of crystal oscillator 3 is compensated, and the oscillation frequency is kept uniform with respect to changes in the ambient temperature. The oscillation frequency of crystal oscillator 3 is dependent upon crystal unit 1 and therefore has a cubic function frequency-temperature characteristic similar to that of crystal unit 1 (see FIG. 2) if temperature compensation is not implemented.
This temperature-compensated crystal oscillator is loaded by, for example, surface mounting on the printed wiring board of a portable telephone, but various other communication circuits are loaded on this printed wiring board in addition to the temperature-compensated crystal oscillator. Of these communication circuits, the AFC circuit supplies the AFC voltage to AFC input circuit 5 of the temperature-compensated crystal oscillator. AFC input circuit 5 applies a voltage that accords with the supplied AFC voltage to voltage-variable capacitance element 6 by way of high-frequency blocking resistor 15 as with the case for the temperature-compensation voltage. Further, of the communication circuits, the AFC circuit receives radio waves from a relay base station, compares the frequency of these radio waves with the oscillation frequency of the temperature-compensated crystal oscillator, and controls the reference oscillation frequency (i.e., nominal frequency) of the temperature-compensated crystal oscillator, for example, the oscillation frequency at 25° C., such that it corresponds with the radio waves from the base station.
In IC 2, voltage-controlled crystal oscillator 3, temperature-compensation circuit 4, and AFC input circuit 5 are driven by a voltage that is obtained from a power supply voltage by way of constant voltage circuit 10 in IC 2. In addition, power supply terminal Vcc, output terminal Vo, AFC voltage input terminal Vf, and ground terminal GND are formed so as to be exposed on one principal-surface of IC 2.
IC 2 is normally provided with protection circuit 11 for preventing electrostatic breakdown of IC 2 due to the application of surge voltage, i.e., an instantaneous high voltage, to the various terminals other than the ground terminal. Protection circuit 11 is a circuit in which, for example, the midpoint of a pair of protection diodes 22a and 22b connected in series is connected to a corresponding terminal and which has its anode connected to the grounding point and its cathode connected to the power supply line, as shown in FIG. 3.
IC 2, in which each of these circuits is integrated as described in the foregoing explanation, and crystal unit 1 are accommodated in, for example, a container for surface mounting and constitute a practical temperature-compensated crystal oscillator. As shown in FIG. 4, a surface mounted container is made up from container body 12 composed of laminated ceramics and cover 13. A depression is formed in the upper surface of container body 12, and steps are formed in the side walls of the depression. Quartz crystal blank 1A that constitutes crystal unit 1 has one end secured to the end of this depression by means of, for example, a conductive adhesive and is thus held inside the depression. In addition, IC 2 is secured by, for example, ultrasonic thermal compression bonding to the bottom surface of the depression. Various mounting terminals 14 which are connected to the terminals of IC 2 such as the previously described power supply terminal Vcc, output terminal Vo, AFC terminal Vf and ground terminal GND are formed on the outer surface of the container.
However, when the temperature-compensated crystal oscillator of the above-described configuration is mounted on a printed wiring board in a portable telephone and put into use, the phenomenon occurs that the frequency stability of the reference oscillation frequency deteriorates. Regarding this point, we investigated the portable telephone as the cause of the problem and found that, although no abnormality occurred during oscillation of the temperature-compensated crystal oscillator while the operation of the portable telephone was halted, the frequency stability deteriorated when the portable telephone was placed in operation and high-level radio waves were radiated from the antenna. Further, when the oscillation frequency of the temperature-compensated crystal oscillator changes, the output frequency (i.e., communication frequency) of a PLL circuit which is set as a multiple n (n being an integer) of this reference signal source also changes, imitating the frequency fluctuation of the reference signal source. Although the AFC circuit in the portable telephone changes the AFC voltage Vf in the direction of oscillation frequency with the reference frequency of the base station to thereby correct the output frequency of the temperature-compensated crystal oscillator when such variation occurs, transitory phase change, i.e., phase error, still occurs. The occurrence of phase error has the unwanted effect of preventing communication, and the occurrence of phase error must therefore be suppressed.